오스템퍼링 열처리한 구상흑연주철의 충격인성에 미치는 Ni 과 Mo 첨가의 영향 = Effects of Ni and Mo Addition on the Impact Toughness in Austempered Ductile Cast Iron

다국어 초록 (Multilingual Abstract)

The microstructural characteristics of ductile cast iron austempered for various times are characterized and correlated to Charpy impact toughness. The desired matrix structure of austempered ductile cast iron consists of a two-phase mixture of carbide-free bainitic ferrite and austenite. During austempering, bainitic transformation can be described as a too-stage reaction : 1) austenite decomposition to bainitic ferrite and carbon-enriched austenite, followed by 2) decomposition of carbon-enriched sustenite to further ferrite and carbide. Therefore, excessive austempering produces a sharp drop in impact toughness and a fracture mode transition from ductile tearing to brittle quasi-cleavage fracture by a carbide formation. Delaying the embrittling reaction by addition of Ni and Mo should allow a greater latitude in austempering time. However, the onset of the embrittling reaction cannot be detected metallographically. By the way, interruption of the austempering reaction by quenching prematurely will result in a formation of martensite. This also reduces impact toughness.